This invention relates to multi-element sensor arrays and in particular to a method and means for equalizing the outputs of the individual sensor elements of such an array.
The method of equalizing the sensitivity of the many sensors of a sensor array comprehended by the invention is particularly applicable to systems that have the function of sensing images in ultraviolet, visible and infrared regions. A typical application would be an array of m .times. n elements in which each element produces an output that is a function of the illumination falling on it. This illumination could be broadband, i.e., ranging anywhere from ultraviolet through infrared; or it could be narrow-band and encompass, for example, only a narrow portion of the infrared spectrum.
The response of this illumination could be photoconducting, photovoltaic, or any other type. Ultimately the response would be converted to an electrical signal (i.e., a voltage level).
A problem that exists in such a sensor array is that while each element will sense the illumination and convert it to an electrical signal (voltage level), the response of the different elements may be different, i.e., the sensitivity is not the same. Moreover, the sensitivity of each element may change with age. It is desirable, therefore, to devise a method of equalizing the sensitivities of the various elements.
For an array that has only a few elements (say of the order of 10 - 100) a simple way of equalizing the response of the several elements would be to use a constant intensity light source (of the intensity that is in the range of interest), illuminate each element in turn with the light source, and, for each element, set the gain of a variable gain transistor amplifier connected to the sensor output to give a constant output. The transistor amplifier output voltage would be measured as the sensor is illuminated with the standard level of illumination. The output of the transistor amplifier bias supply would then be varied, thereby varying the gain of the variable gain amplifier until the output level has reached a standard value. This standardization procedure would be repeated for each of the sensor elements.
While this technique is satisfactory for an array of discrete elements that are relatively few in number, it is of course out of the question for an integrated array of many (say 100,000) sensor elements. It is the standardization of such an integrated multi-element sensor array system to which the invention addresses itself.